Method for fabricating optical devices by assembling multiple wafers containing planar optical waveguides

ABSTRACT

A method for fabricating optical devices comprises the steps of preparing a first substrate wafer with at least one buried optical waveguide on an approximately flat planar surface of the substrate and a second substrate wafer with at least a second buried optical waveguide. The waveguides so formed may be straight or be curved along the surface of the wafer or curved by burying the waveguide at varying depth along its length. The second wafer is turned (flipped) and bonded to the first wafer in such a manner that the waveguides, for example, may form an optical coupler or may crossover one another and be in proximate relationship along a region of each. As a result, three dimensional optical devices are formed avoiding conventional techniques of layering on a single substrate wafer. Optical crossover angles may be reduced, for example, to thirty degrees from ninety degrees saving substrate real estate.

This application is a Divisional application of Ser. No. 10/362,954filed Feb. 26, 2003 a National Filing pursuant to 35 U.S.C. 371 basedupon International Application No. PCT/US01/27393, filed Sep. 4, 2001,which claims priority to U.S. Provisional Application No. 60/230,205,filed Sep. 5, 2000.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the field of fabricating opticaldevices and, more particularly, to a method for fabricating opticaldevices, including, for example, couplers and crossovers, byindividually assembling multiple wafers, each containing at least oneplanar optical waveguide (POW), and bonding the wafers together to formsaid optical devices and the optical devices fabricated by such a methodof fabricating.

2. Description of the Related Arts

The defining element of planar optical waveguide (POW) technology is theoptical waveguide itself, a region of relatively high refractive indexwhich is typically much longer than it is wide or deep, formed, forexample, by diffusion on a substrate wafer of lower refractive indexmaterial. Light is bound to the high-index guide, eliminating thenatural diffraction spreading of a free light beam and allowing thelight to be turned through curving paths. Typically, the waveguides arefabricated near the (approximately planar) surface of the substratewafer, like the interconnections in an electronic integrated circuit.

Current planar waveguide technology has been focused on a single layerof waveguides, which, for example, severely limits crossing ofwaveguides. A few research results have explored three dimensionalcouplers in semiconductor materials, but that work requires multipleepitaxial growth processes of layers on a single wafer, an expensiveprocess which has not been extended to glass waveguides. Presently, asmany as seven layers may be grown on a substrate with differentproperties, but with each additional layer, complications in tolerancesresult. Recently, Raburn et al. in their paper Double-Bonded InPlInGaAsPVertical Coupler 1:8 Beam Splitter, published in “IEEE PhotonicsTechnology Letters”, Vol. 12, December, 2000, disclose a three layerdouble bonding process where the three layers are grown using vapordeposition. A first wafer with etched waveguides is bonded to a blanksecond wafer. The second wafer is then etched and a third wafer bonded.A waveguide of one layer may be at a different height than anotherwaveguide of another layer.

Direct wafer bonding processes are known, for example, from Zhu et al.,Wafer Bonding Technology and its Application in Optoelectronic Devicesand Materials, published in “IEEE Journal of Selected Topics in QuantumElectronics,” vol. 3, pp. 927–936, 1997 and from Black et al., WaferFusion: Materials Issues and Device Results, published in IEEE Journalof Selected Topics in Quantum Electronics,” vol. 3, pp. 943–951, 1997.

Consequently, there remains a need in the art for improved methods offabricating optical devices and, in particular, couplers and cross-overshaving different properties than conventional optical devices.

SUMMARY OF THE INVENTION

We propose to individually assemble and bond two planar waveguidesubstrate wafers together according to the desired optical device. Thesurfaces of the individual substrate wafers are placed and then bondedface-to-face according to known bonding techniques to fabricate thedesired optical device so that the waveguides are in appropriate,proximate relationship to one another depending on the device to befabricated. Where waveguides cross each other at an angle forming acrossover, an interconnection crossover between waveguides results, withmuch better isolation and lower crosstalk than the crossthroughsachieved in conventional single-wafer technology. Also, it will be shownthat wafer real estate may be preserved in crossovers as the crossoverangle may be substantially reduced from, for example, ninety degrees toa substantially lesser angle such as thirty degrees.

Where waveguides are at least in part constructed with parallel segmentson the substrate wafers, for example, forming a coupler, overlap ofoptical modes creates a three-dimensional (3D) optical coupler linkingthe upper waveguide of one substrate wafer and a lower waveguide of asecond substrate wafer. (By parallel segments is intended theconstruction of waveguide segments in a line whereby the two linesegments are in the same plane and the waveguide segments are opticallycoupled.) By appropriate design of the waveguides and one or moreinteraction regions, the energy transfer may be complete or partial,wavelength-selective or broadband, polarization-selective orpolarization-independent. The proposed method of fabrication mayfabricate couplers between dissimilar waveguides, such as a pump couplerwhich directly connects an undoped waveguide carrying pump light to anErbium (Er)-doped waveguide which amplifies signal light.

The invention, for example, will enhance the optical transport androuting systems at the heart of a telecommunications network to be madesmaller, cheaper, more reliable and more capable. In particular,Er-doped waveguide amplifiers (EDWAs) fabricated according to theproposed method may replace bulkier Er-doped fiber amplifiers intransport systems in which three dimensional couplers fabricatedaccording to the present invention may be used to improve the gain,power efficiency, and wavelength flatness of existing EDWAs. ImprovedEDWAs should also play an important role inside optical switches androuters, to overcome the loss of passive components. Three dimensionaloptical couplers may also serve as channel selection and noise reductionfilters inside highly integrated DWDM systems. Also, the high-isolationcrossovers enabled by the proposed wafer bonding process may enablesophisticated routing photonic integrated circuits (PICs), eliminatingmuch of the fiber congestion and complexity associated with backplanesand front panels of current DWDM equipment. The present invention willallow PICs to be simultaneously more compact and more capable. Also, thephotolithography needed for fabrication of 3D couplers by wafer assemblyis less demanding than that needed for single layer couplers. Finally,the creation of couplers between dissimilar waveguides, difficult orimpossible with single-layer techniques, is practical with waferassembly. Glass or silica waveguides, which are preferred for EDWAs andmost passive devices, are easily accommodated by the present technique.

Planar optical waveguide (POW) technologies and fabrication inaccordance with the proposed process enable a broad range of opticaldevices by routing light along controlled paths near the surface of asubstrate wafer. In some cases, the routing is simple, such as aFabry-Perot laser, which guides light back and forth through anamplifying region until feedback causes a controlled oscillation. Inothers, the routing may be quite complex, involving switching and/ormultipath interferometry. Although similar functions can often beachieved with fiber- or free-beam technologies, planar waveguideprovides the most compact implementation of complex devices, with thegreatest mechanical stability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) shows a conventional diffusion method of fabricating a planaroptical waveguide on a substrate wafer, depth of the waveguide 15 beingdetermined by diffusion time, temperature, electric field density andother factors; FIG. 1( b) shows the result of burying a waveguidesegment more deeply in the center of a substrate wafer than the pointsof entry at either end of a waveguide.

FIG. 2 show three dimensional wafer bonding assembly according to thepresent invention, for example, for fabricating a waveguide crossoverwherein FIG. 2( a) shows the separate preparation of two wafers eachhaving a waveguide diffused thereon, and a direction of flipping tofabricate a three dimensional crossover shown in isometric view whileFIG. 2( b) shows the additional step of etching recessed areas in thevicinity of each crossing waveguide to improve isolation and crosstalkperformance.

FIG. 3 shows a conventional process for fabricating a simple two-modeoptical coupler utilizing single-layer planar optical waveguidefabrication wherein FIG. 3( a) provides an isometric view and FIG. 3( b)a top view.

FIG. 4 shows a three dimensional wafer bonding assembly according to thepresent invention, for example, for fabricating a three dimensionaloptical coupler wherein FIG. 4( a) shows, for example, the diffusion ofidentical waveguides on identical substrates and a flipping of one on tothe other, forming the isometric view (FIG. 4( b)) and top view (FIG. 4(c)) of a three dimensional coupler fabricated according to theprinciples of the present invention.

FIG. 5 shows the fabrication of an optically triggered waveguide switchin accordance with the present invention.

FIG. 6 shows construction of a typical crossover angle of approximatelyninety degrees in FIG. 6( a) and how a substantially lesser angle, forexample, a thirty degree crossover angle may be achieved and wafer realestate saved in FIG. 6( b) utilizing a variant of the method offabricating an optical crossover of FIG. 2; FIG. 6( c) is an alternativeembodiment in cross-sectional view of a device fabricated according tothe present invention in which waveguides are implanted having varyingdepth in the planar substrate as taught in FIG. 1( b), thus forming acrossover of waveguide segments by varying the depth of theirfabrication. In the design of FIG. 6( c), a substantially reducedcrossover angle may also be achieved.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1( a) shows a conventional diffusion method of fabricating a planaroptical waveguide 15 on a substrate wafer 10, the depth of the waveguide15 being determined by diffusion time, temperature, electric fielddensity as well as other known factors. A defining element of planaroptical waveguide (POW) technology is the optical waveguide itself, aregion 15 of relatively high refractive index which is typically muchlonger than it is wide or deep, surrounded by wafer substrate materialof lower refractive index. Light is bound to the high-index guide 15,eliminating the natural diffraction spreading of a free light beam andallowing the light to be turned through curving paths as necessary.Typically, the waveguides are fabricated near the (approximately flatand planar) surface 12 of a substrate wafer 10, like theinterconnections in an electronic integrated circuit. (However, per FIG.1( b) and FIG. 6( c), some embodiments may involve burying thewaveguides more deeply, for example, at the center of the substratewafer than at the points of entry to intentionally build a crossover orother optical device.) Waveguides 15 may be fabricated, for example, bydeposition and etching, ion implantation, diffusion techniques, or otherknown techniques, and the materials forming the wafer substrate 10 maybe semiconductors, crystalline dielectrics, glasses, or polymers, amongother known materials. Although many implementations are possible, onepreferred embodiment may comprise diffused waveguides in glass, aneconomical technology with some specific advantages discussed furtherbelow.

Following from left to right in FIG. 1( a), formation of diffusedwaveguides 15 begins with preparation of a substrate wafer 10 withcontrolled properties at surface 12. Patterned areas (typically straightor curved stripes) of a diffusion source material 13, which may bedopant, are deposited onto the approximately flat, planar surface 12.Then the composite structure 10 is subjected to heat and/or an electricfield in a controlled manner, allowing dopant ions to diffuse from thesource layer into the substrate 10 and producing regions of increasedrefractive index where the diffusion source overlay 13 is present. Thewaveguide depth and the lateral spreading at the pattern edges aredetermined by the appropriate choice of materials and processconditions. Electric fields may be applied during the process to obtainmore flexible control of vertical and horizontal dopant profiles. Dopantin-diffusion is a simple process, adaptable to small- or large-scaleproduction of optical waveguides 15 and has been commercially applied tolithium niobate optical modulators. Other suitable processes may beemployed to advantage depending on the desired optical device. At theright of FIG. 1( a) is shown the wafer 10 with waveguide 15 implantedbut with dopant source material still present. Finally, the dopantsource material is removed and the top surface planed leaving the wafer10 complete with planar optical waveguide 15 implanted. That is, thewaveguide lies in a plane parallel to the approximately flat planarsurface of the substrate wafer. As already indicated, the finishedproduct is shown in FIG. 1( a) with a straight waveguide 15 appearing tobe at even depth. In fact, by varying the initial location of dopantsource material, degree of heat and field intensity, curvature and depthand other measurable characteristics of the formed waveguide 15 may bevaried as shown in FIG. 1( b). In this figure, the waveguide isintentionally buried more deeply in the center of the wafer withsubstrate material covering the waveguide in the center. Thus, thewaveguide may follow a, for example, vertically curved path through thesubstrate.

Referring to FIG. 2, there is shown the process of fabricating, forexample, a three-dimensional optical crossover according to the presentinvention which employs the conventional processes of FIG. 1 inpreparing at least first and second wafer substrates. Moving from leftto right in FIG. 2( a), a wafer substrate 10 has diffused or otherwisedeposited therein at least one waveguide 15 formed on or near preferablyplanar surface 12. (Similar reference numerals have been used betweenFIGS. 1 and 2 to refer to similar elements since FIG. 2 relies in parton POW waveguide formation as per FIG. 1). A second substrate wafer 20may be similarly formed having one or more waveguides 25 diffused orotherwise implanted therein. The first wafer substrate 10 may be thenused as a stationary substrate for receiving a flipped wafer substrate20 as a second layer of a completed device to form a completed threedimensional optical crossover 30 (shown to the right of FIG. 2( a). Inan alternative embodiment and to improve crosstalk isolation and perFIG. 2( b), recessed areas 18 and/or 28 may be formed in one or theother wafer substrate 10 or 20 or both prior to flipping and bonding. Iftwo recessed areas 18, 28 are formed, they may preferably match afterflipping. When a recess is formed in one or the other or bothsubstrates, the recess(es) will form a void which may be filled tocontain inert gas, a vacuum or other desired medium as will be discussedfurther below. As will be further discussed in connection with FIG. 6,the crossover angle may be reduced from ninety degrees to a much smallerangle, for example, thirty degrees, more or less, using the proposedmethod of fabrication of the present invention saving wafer real estate.

A key building block of POW devices is the conventional two-mode couplershown in FIG. 3. At the opposite edges of the substrate, where the twowaveguides 32 and 34 are well-separated on substrate wafer 38, there isnegligible overlap of the optical modes, and no energy is transferredfrom one waveguide to the other. In contrast, mode overlap is strongerin a central interaction region 36, leading to coupling of the waveguidemodes and a predictable shift of light from one waveguide to the other.By appropriate design of the waveguides and the interaction region, theenergy transfer may be complete or partial, wavelength-selective orbroadband, polarization-dependent or not. If the interaction region canbe controlled by a parameter such as temperature, application of lightor electric field, a switch may be realized. In most POW devices, theseoptical couplers are laid out laterally, between two waveguides at thesame constant depth relative to the planar surface. The fabricationprocess of FIG. 4, as discussed below, provides an alternative, verticalor three-dimensional (3D) coupler in which not all waveguides fallwithin a single plane but are closely proximate to one another and maylie in proximate parallel planes.

Vertical or 3D couplers in semiconductor materials have been constructedpreviously by fabricating two layers of waveguides on a singlesubstrate. Up to seven layers are known. In contrast, we propose thattwo or more substrates, each prepared with a single layer of waveguides,be juxtaposed so that the optical modes of waveguides on differentwafers overlap with each other. A practical, stable means of assuringthe required intimate contact is the use of direct wafer bonding asintroduced in the Description of the Related Arts section above or inaccordance with other well known bonding techniques. The process may usewafer substrates with smooth, clean surfaces that may be treated withheat and pressure to form a strong, adhesive-free bond. Wafer bondinghas been demonstrated with a wide variety of matched and mismatchedsubstrates, including commercial fabrication of silicon-on-insulatorwafers. The proposed assembly is illustrated in FIGS. 2 (cross-over), 4(coupler) and 5 (a waveguide switch) among other possible devices.

Referring to FIG. 4, there is shown a method of fabricating a threedimensional optical coupler. A first substrate wafer 200 has formedthereon a first curved waveguide 210 along the approximately flat,planar surface of wafer 200. A second identical wafer 220 has a secondwaveguide 230 formed therein in the same manner. Of course, the formingof identical waveguides on identical wafers has some economic advantagesover forming differently shaped waveguide(s) on the second wafer, butsuch is possible within the principles of the present invention. As inFIG. 2, the second wafer 220 is flipped so that the formed waveguidesare in close proximity after flipping and bonded to the first wafer 200forming a completed three dimensional coupler shown in isometric view inFIG. 4( b) and in top view in FIG. 4( c) having a central couplingregion.

The depicted, wafer-bonded 3D coupler of FIG. 4( b) or (c) can enable awide variety of novel and capable devices. One major advantage of theprocess is its ability to construct couplers between radically differentwaveguides, rather than identical waveguides as shown. For example, thewaveguides 210, 230 on the two substrates 200, 220 may be derived fromdifferent dopant ions or treated at different diffusion temperature orelectric field, to obtain different optical mode shapes or propagationvelocities. The implanted waveguides may lie in parallel planes in thecompleted coupler device or not as the designer chooses. For example,the designer may vary the depth of implantation of the waveguides 210,230 along their length per FIG. 1( b) or their paths through thesubstrate, for example, one curved, one straight or varying thecurvature of the separate waveguide paths. The substrates into whichwaveguides are diffused may also differ. An Erbium (Er) doped waveguideamplifier (EDWA) constructed from one Er-doped waveguide, for example,waveguide 210, and one passive Er-free waveguide, for example, waveguide230, is a useful instance of this idea. The active Er-doped waveguidecan be optimized to amplify the signal light, while the Er-freewaveguide is independently optimized to transport pump light with verylow loss. Unlike conventional end-pumped EDWAs, the wafer-bonded EDWA ofthe present invention can be designed with multiple pumping locationsinside the length of the active region, to assure good noise figure andefficient pump utilization at all points in the amplifier. The use of 3Dcouplers according to the present invention is also valuable indistributing pump power to arrays of waveguide amplifiers and incombining power from redundant pumps, due to the enhanced crossovercapabilities discussed below.

In addition to its use for pump light routing, the 3D coupler of FIG. 4can also contribute wavelength shaping filters to EDWAs. A coupler whichextracts light at the spontaneous emission peak can be used to equalizethe amplifier's gain spectrum, providing any tailored specific shapedesired, for example, the flat wavelength response needed forwavelength-division multiplexed systems. Mid-amplifier filtering usuallyprovides the best noise and power efficiency, and a 3D coupler accordingto FIG. 4 offers the unique opportunity of filtering in the mid-stage ofthe amplifier without breaking the active waveguide.

There are several ways in which 3D couplers constructed according to theprinciples depicted in FIG. 4 may enhance the performance of wide-bandEDWAs, including compound amplifiers which cover both C-band (typicallyfrom 1525–1565 nm) and L-band (typically from 1565–1615 nm). First, 3Dcouplers may be used to inject pump light from pump lasers at two ormore wavelengths. Second, 3D couplers may be placed to inject pump lightat strategic points along an active waveguide which has varying dopantcomposition along its length. Third, wavelength-selective 3D couplersmay be used to route C-band and L-band signals to the appropriate pathsin a compound amplifier.

In alternative embodiments, one (or more than one) optical crossover mayenable distribution of a single pump source via optical coupling to notonly one amplifying waveguide but two or more amplifying waveguides. Oneof ordinary skill in the art may construct a compound, complex devicehaving signal distribution among waveguides only limited by theimagination.

Mating of active and passive waveguides can also enable an opticallytriggered waveguide switch, as shown in FIG. 5. The active waveguides Band D, 60 and 62, are doped with saturable absorber ions. These can bepumped to a higher energy state by optical injection at the resonantwavelength, which will saturate the absorption and shift the refractiveindex at non-absorbing wavelengths. Thus, if waveguide B 61 is opticallypumped at its resonant wavelength, its refractive index at longer,transparent wavelengths will be lower than that of unpumped waveguide D62. Thus, long-wavelength light from input waveguide C 52, a passivewaveguide, will be preferentially coupled into waveguide D 62.Subsequent coupling to passive waveguide E 53 can be used if it isdesirable for the output waveguide to be undoped. If the optical pumpingis moved from waveguide B 61 to waveguide D 62, the signal output can beswitched to output waveguide A 51 from waveguide E 53.

Another advantage of 3D couplers is flexible and reproducible tailoringof the coupling strength between the coupled waveguides. In aconventional lateral coupler per FIG. 3 built from a single layer ofwaveguides, the coupling strength is very sensitive to any errors inwaveguide spacing. At the same time, lithography and patterning are mademore difficult by the close approach of multiple waveguides. Incontrast, the 3D coupler of FIG. 4 allows independent lithography andpatterning of the two waveguides 210, 230, while reducing sensitivity tolateral placement errors. Realizing the 3D coupler by wafer bonding ofdiffused waveguides offers an additional degree of customization: thecoupling strength can be tuned, either before or after wafer bonding, byadjusting the diffusion parameters to control waveguide depth as shownin FIG. 1( b).

Three dimensional waveguide couplers per FIG. 4 also offer powerfulcapabilities for crossovers in photonic circuits. In conventionalsingle-layer technology, waveguides cannot physically cross over oneanother, they must cross through each other, introducing crosstalk andloss in both paths. Although these impairments can be made manageable byrelying on large intersection angles, the resulting circuits consume alot of expensive real estate on the wafers (FIG. 6 a). FIG. 6( a) showsa conventional crossover at ninety degree angles and how wafer realestate is wasted. By coupling light into a second layer for thecrossover, it becomes possible to achieve low crosstalk and loss withmuch smaller crossing angles, for example, on the order of thirtydegrees, yielding a more compact, cost-effective photonic circuit (FIG.6 b) using far less wafer real estate.

For even more compact circuits, it may be desirable to arrange weakercoupling for the crossover areas than for the coupler areas. There aretwo possibilities for achieving this area-selective coupling strength.First, waveguides may be vertically curved by varying depth of implant(or curved along the horizontal planar surface of each substrate); thatis, the waveguides may be implanted deeper (weaker coupling) in someregions than they are in others of the substrate wafer. FIG. 6( c) is analternative embodiment in cross-sectional side view of a devicefabricated according to the present invention in which the waveguidesare implanted in each substrate wafer at variable depth. Variable-depthwaveguides may be fabricated in each substrate by applying localizedelectric fields during diffusion, by applying localized regions ofhigher temperature (such as by laser heating), or by localized ionimplantation to accelerate the diffusion process. In FIG. 6( c), acrossover angle of substantially less than 90 degrees, for example,thirty degrees can also be achieved. Alternatively, coupling may beweakened in regions of waveguide crossover simply by creating, forexample, a rectangular recessed area 18 or 28 in one or both substratesbefore wafer bonding, as shown in FIG. 2. After assembly as shown ineither FIG. 2, the crossover areas will have local voids between theupper and lower waveguides. The depicted recessed areas may haverectangular or curved shape and, if rectangular, are of even depth, butmay be of other shape and depth according to desired results. Whetherfilled with vacuum, air, or an inert gas, the voids formed by recessedarea(s) 18, 28 and as shown in FIG. 2 will have a refractive index muchlower than that of the glass substrates, ensuring an effectivedecoupling in the crossover areas and improved crosstalk performance.The recessed areas may be intentionally filled with another material orcomposition of solids, liquids and/or gases that may have an opticalfiltering or other attenuating effect at one or more wavelengths.

As a final note, it may be understood by one of ordinary skill in theart that the wafer-bonded 3D coupler technology is not limited to twolayers of waveguides. As has been noted in other types of wafer-bondeddevices, the bond is very strong, and it is quite practical to polishaway the body of the upper substrate, and then to bond on a third wafer,(which may be flipped or not flipped before bonding) to obtain morecomplex composite structures. Moreover, other embodiments and otheroptical devices than those described above may be formed using theprinciples of the present invention simply described in the drawings asa flip and bond assembly of planar optical waveguide substrate wafers.All articles referenced herein should be deemed to be incorporated byreference as to any subject matter deemed essential to an understandingof the present invention and/or conventional waveguide diffusion and/orwafer bonding technology.

1. An optical amplifier apparatus, comprising: at least one waveguidecapable of optical gain; at least one optical coupler capable ofextracting light from said amplifying waveguide without interrupting orterminating said amplifying waveguide wherein said at least one opticalcoupler extracts light at selected wavelengths to achieve an equalizedgain spectrum of predetermined shape for an optical amplifier.
 2. Theapparatus of claim 1 in which said gain spectrum of predetermined shapecomprises a wavelength flattened shape.
 3. An optical switch apparatuscomprising at least one waveguide capable of saturable absorption,coupled to at least one optical coupler comprising at least one wafercontaining at least one buried optical waveguide assembled and bonded toat least one other wafer containing at least one buried opticalwaveguide, the bonded wafers being positioned with respect to oneanother and forming said optical coupler such that different regions ofeach waveguide are formed by the bonding to be in optical relationshipto one another.
 4. An optical switch apparatus comprising at least oneoptical coupler comprising at least one wafer containing at least oneburied optical waveguide assembled and bonded to at least one otherwafer containing at least one buried optical waveguide, the bondedwafers being positioned with respect to one another and forming saidoptical coupler such that different regions of each waveguide are formedby the bonding to be in optical relationship to one another, and whereinthe refractive index of at least a portion of one waveguide iscontrolled to determine routing of at least one optical signal throughthe switch.
 5. An optical switch apparatus as recited in claim 4,wherein a control is provided optically.
 6. An optical switch apparatusas recited in claim 5, wherein said optical control is provided by atleast one waveguide.
 7. An optical switch apparatus as recited in claim4, wherein a control is provided electrically.